Optical fibers offer many advantages over traditional wires in communications networks. Data is encoded by modulating an optical signal that is transmitted along the optical fiber to a receiver. The data is subsequently recovered through demodulation of the received optical signal.
Due to the nature of the optical signal and the transmission medium, communication through the use of optical pulses is preferred over phase, amplitude, or frequency modulation of a sinusoidal carrier. One such encoding technique frequently used to communicate analog information in a public switched telephone network is pulse code modulation. Pulse code modulation uses groups of pulses in specific patterns to represent individual digital values. PCM digitally encodes the sampled analog data such as speech by providing a pulse sequence corresponding to a digitized sampled value.
One disadvantage of PCM is that lookup tables are required for encoding and decoding circuitry. Another disadvantage is that a frame sizeable enough to accommodate the largest pulse sequence is required to communicate each code. A four bit optical PCM frame is limited to representing one of sixteen values despite the ability of electronic equipment to switch faster and thus sustain greater data capacity in the same time frame.